Your company can't afford a dedicated leased line for connecting various branch offices. They however, want to grant safe, controlled access to the company’s network for telecommuters, and branch offices via the untrusted Internet. For setting up the controlled network, they need a program that is free of cost, open source, easy to administer, secure, and platform-independent.

Given all communications will be done over untrusted networks, which cryptographic protocols can I use to protect against eavesdropping, message forgery, tampering and also provide authentication?

I will also need to connect Linux and Windows Clients. Stepwise, how can I accomplish that?